The present invention relates to a flexible tubular pipe which can be preferably used in deep-sea applications, for depths of between 1000 and 3000 m, although it can also be used for depths of less than 1000 m. Such flexible tubular pipes are used in subsea oil production installations for transporting fluids such as hydrocarbons.
Several types of flexible tubular pipes are used at the present time and are described in API (American Petroleum Institute) 17 J. In certain flexible pipes, there is a pressure vault which consists of a helical winding with a short pitch of a shape to wire which may be self-interlockable or interlockable by means of a fastener. Likewise, the metal carcasses used in flexible pipes called “rough bores” are formed from a crush-resistant doubly interlocking profiled metal strip.
In all cases, it has been attempted to improve the moment of inertia/weight ratio of the interlocked strips or shaped wires used for producing the various metal layers of the flexible pipes.
For deep-sea applications and in the case of pressure vaults, the reinforcing wires must have a high moment of inertia in order to withstand the external pressure and a low weight in order to reduce the total weight of the flexible pipe so as to improve the performance of the pipelaying means and allow the flexible pipe to be self-supporting.
Several solutions have been proposed.
A first solution has consisted in using a shaped wire, the cross-section of which is in the form of an I, as described in FR-A-2 782 142. Such a shaped wire has an acceptable moment of inertia/weight ratio but the manufacturing cost is very high because of the fact that it is obtained by rolling or wire drawing.
The profiles shown in the figures must not exceed a certain width L so that the hollow profile can be wound helically. This is because if the profile is too wide, during winding the forces will be too high and there would be a risk of melting the side walls of the profile buckle. The profile must therefore have a maximum width L which depends, on the one hand, on the thickness e of the strip and, on the other hand, on the height H of the profile. Tests carried out have shown that the results are satisfactory when:0.5<L/H<4;L/e>20
In Patent FR 2 665 237 it is recommended to produce a tubular metal carcass comprising at least one box section wound in a helix with a short pitch, the said metal carcass being obtained by means of two complementary profiled strips wound helically with a short pitch. Many examples of profiled strips are described and shown in that document, some of which, such as for example those in FIGS. 8 and 9, consisting of a strip in the form of an elongated S and having a box section at a first end and an upwardly curved fastening edge at the other end, the fastening edge penetrating a dish formed by the box section and the transverse bar of the S of the preceding turn. The curved fastening edge may rest on the bottom of the dish (FIG. 9) or it may not be in contact with the said bottom (FIG. 8).
It should be noted that all the cross-sections of the box sections provided at one or both ends of each profiled strip are square or rectangular cross-sections.
Although such box-section profiled strips have been satisfactory, they have been found to have certain drawbacks. When one considers that a box section is formed by parts of the same turn of the profiled strip and comprises an upper wall, a lower wall and side walls and when an external force is applied to one of the upper and/or lower walls, such as a compressive or crushing force or else a force generated by the pressurized fluid, buckling of the side walls or faces and/or the upper or lower walls of the box section may then occur, thereby reducing, at least locally, the crush resistance of the internal carcass.
For forces or pressures exceeding a certain value, buckling of the side walls of the box section occurs.
For lower forces or pressures and when the side walls are not strictly perpendicular to the upper and lower walls of the box section, crushing of the said box section may occur, resulting in the side walls moving further apart or closer together (opening or closing of the box section). To prevent this opening or closing phenomenon, it has been recommended in certain cases such as, for example, in FR 2 665 237 to weld one end of the box section at one point on the strip. However, such a solution increases the manufacturing cost and it is difficult to implement.
In a prior embodiment, the box-section strip adopts the shape of a pair of spectacles. This profile is produced from a strip whose free edges are welded to this same profile so as to give the profile good stability for laying. However, the welding operation is difficult to implement and considerably increases the manufacturing cost. Because the welds are off-centre with respect to the neutral fibre of the profile (at the radial position where it is neither compressed nor stretched during winding), they are subjected to high stresses during the spiralling. This may result in a local fracture of the weld. Such a fracture of the weld may, in turn, result in a considerable reduction in the mechanical performance of the pressure vault, of around 10 to 30%, because of the fact that the profile is no longer stable.
In another prior embodiment, the spectacles-shaped profile is similar but produced from two tubes, of square cross-section, the said tubes being joined together by a strip which is welded to these tubes. The welding operations again considerably increase the manufacturing cost.
In addition, these various shapes of the profile do not make it possible to limit the creep of an adjacent sealing sheath. It is sought to prevent such creep as far as possible, mainly in the case of the pressure vault. It should also be noted that no information is provided regarding the width/thickness ratio of the strip, although this has become important in order to ensure stability of the profile during pipe laying, so as to prevent the buckling effect.
This is because if the profile has too large a width with respect to its thickness, the side walls of the box section will buckle during spiralling or winding with a short pitch.
In French Patent No. 2 808 070, the profiles that are described give good results but sometimes have drawbacks, especially because of the great dissymmetry of the profile, this dissymmetry constituting an obstacle during spiralling, which is consequently difficult to carry out. In addition, the moment of inertia/pitch ratio is reduced by the presence of the free edge which provides little moment of inertia.